Hi Stan, Your question is really quite interesting, as high precision by high cost is easy to achieve, but the real art is to do it by low cost. In this sense, I would like to give you some really practical hints. The company's division I'm working for, produces automotive electronics, i.e. instrument clusters with a "precise" clock, i.e. < 1 min /month deviation over a temperature range of -40 to +85°C, giving +/-50ppm overall. We have production volumes of 100k-300k/yr. per application, or several 10Million applications for all of our customers. High and longtermed reliability / quality in production and field and ridiculously low costs are mandatory. We cannot use external oscillator circuitry, hardware trimming, TCXO or temperature compensating methods for those reasons.
We use embedded controllers; the oscillator consists of an internal CMOS inverter, 2 ceramic caps 5-10% and an AT-cut, fundamental crystal with clock trimming by software correction. SMD components are mandatory, also for cost reason, so I recommend an AT51-GW (gullwing) type crystal, which is an AT51 / CS10 / HC49 (supplier dependant) case with bended leads and an additional metal clamp for mounting. Other cases with plastic plate underneath are more expensive. Price for the crystal should be less than 20 Cents, depending on further requirements. I highly recommend NDK ( [1]www.ndk.com), they produce the quartz blanks on their own, and also design nice OCXO's , so they have a lot of technological experience. Vectron (formerly Corning, formerly Telequarz) or Kinseki as 2nd sources, also have automotive experience. Ask for tighter automotive specs, but lower price, as ours depend also on our automotive quality requirements (e.g. AEC Q200 and more plus tough warranty regulations). 27MHz can be realized by fundamental crystal, but it's on edge of technology because the crystal is pretty thin already. Therefore, a tighter specification is critical and costly, so use PLL design with 9MHz or less instead! We often have discussions between the (digitally minded) designers of the inverter and the crystal supplier: For us, an additional critical aspect for series production are oscillation start margin (which is very crudely checked by our hardware designers) and ESR dips over temperature. Crystal quality (e.g. residual silver particules from electrodes) vs. input/output impedance etc. of inverter are to be observed. Due to cost and practical reasons, we also do not deal around with simulations or excessive measurements. I also confirm , that SPICE simulations etc. are not applicable here. Instead, we use cost free service/knowledge of suppliers. We send a sample board to NDK, they measure the circuitry and propose values of C1, C2 and the nominal center frequency of the crystal they will deliver. Perhaps they know your type of inverter already, as they have a huge database. Takes about 4-6 weeks. It's not possible to choose other parameters freely anyhow, in contradiction what you assumed, as nominal frequency, case / crystal blanket size and oscillator determine the overall parameters! Typical specification for a 16MHz crystal over automotive temp. range of -40..+85...+125°C are: +/- 20...50ppm freq. deviation at room temperature, 50µW additionally 20..50ppm over temperature (20 ppm will double the price compared to 50ppm) 50µW-1mW max. power 5ppm/yr. aging ESR and Dips < 80 Ohm over temp. I think that you have to do some qualifications of your product on your own to safeguard your requirements. Your initial precision and also the long term aging requirements are hardly to be achieved by design or specification only. On the other hand, it depends on your or your purchasings' negotiation skills, what your supplier is willing to specify, as you for sure need tighter specs. Best regards Dr. Frank Stellmach, Germany References 1. http://www.ndk.com/ _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
